1. Field of the Invention
This invention relates generally to improving output and efficiency of combustion engines and, more particularly, to cooling inlet air to combustion turbines.
2. Description of the Background
Internal combustion engines, including gas turbine engines, also known as combustion turbines, are means of converting fuel such as natural gas, oil, or other liquid or gaseous fuels to electrical, mechanical and/or other forms of useful power output. The available power output of internal combustion engines, however, typically diminishes with rising inlet air temperature. Typical combustion turbines, whose power output is defined as 100% at 15 C. (59xc2x0 F.) are derated to approximately 85% power output at inlet air temperatures of 90 to 100xc2x0 F. Conversely, cooling in the inlet air to 40 to 50xc2x0 F. results in an increased rating of approximately 105% power output. Accordingly, it is of interest, and has been put into practice, to cool the inlet air to combustion turbines, during times of hot ambient air temperatures, to increase the power output. In particular, inlet air cooling is advantageous during periods when ambient air temperatures are high and concurrently the demand for, and the value of, power output is high.
Among the several methods that have been traditionally used for combustion inlet air cooling are the following, each with inherent limitations, as noted.
Although quite low in unit capital cost, that is, capital cost per unit of incremental power output, evaporative cooling can only cool the air to a temperature near the ambient wet bulb temperature. Therefore, this method has quite limited value in terms of power output enhancement, especially on hot, humid days when it is most needed.
Absorption cooling is able to be powered by heat recovered from the combustion turbine exhaust, however, this method has relatively high unit capital cost and some limitations on how low a temperature can be achieved, for example approximately upper 40s to 50xc2x0 F. air, due to absorption chiller operation in the lower 40s.
Although mechanical refrigeration has no temperature limitations, it has relatively high unit capital cost and consumes roughly one third of the incremental combustion turbine output in parasitic power consumption when operating to produce 40 to 50xc2x0 F. inlet air.
Chilled water thermal energy storage offsets the parasitic power losses of mechanical refrigeration to off-peak periods, when energy value is low. Also, it cuts unit capital cost dramatically. It has, however, a temperature limit for cooling the air, which is not lower than middle to upper 40s xc2x0 F., due to the temperature limit of chilled water storage that is set by the point of maximum water density, 4.1 C. or 39.4xc2x0 F. Also, the storage tank volume requirements are large.
Although ice thermal energy storage can achieve lower air temperatures, for example upper 30s to 40xc2x0 F., it is more complex and costly than is chilled water storage. Ice thermal energy storage can achieve greater combustion turbine output enhancement, but the incremental output versus chilled water thermal energy storage is at a very high capital cost. Installations typically employ weekly-cycle ice harvester technology, resulting in storage tanks that are as large as the daily cycle chilled water storage tanks.
Thus, it would be desirable to achieve a means of cooling combustion turbine inlet air to a temperature level as cold, or colder than, is practical when using ice thermal energy storage, yet with the simplicity and low unit cost associated with the conventional chilled water thermal energy storage. Also, it would be desirable to achieve a means of reducing the excessive storage volume requirement associated with the conventional chilled water and ice thermal energy storage systems.
Accordingly, it is an object of the present invention to provide a method and an apparatus for improving and enhancing the power output and efficiency of combustion turbine engines that can eliminate the above-noted defects inherent in the prior art.
Another object of this invention is to provide an improved method and apparatus for cooling inlet air to a combustion turbine by using chilled fluid thermal energy storage.
It is a further object of this invention to provide cooled inlet air to a combustion turbine using chilled fluid thermal energy storage in which the chilled fluid is an aqueous solution of a salt, such as sodium chloride an calcium chloride.
According to one aspect of the present invention, an improved system and method for cooling the inlet air of a combustion turbine or other internal combustion engines is provided, in which thermal energy storage system is employed using undersized chillers that is, smaller than the peak inlet air cooling load, which operate during relatively lengthy off-peak periods to charge a stratified thermal energy storage system that is in turn discharged and used to cool the inlet air during relative brief peak periods. The present invention is an improvement over other combustion turbine inlet air cooling systems in that it employs a thermally stratified system, but not one storing essentially pure water as is the case with conventional chilled water thermal energy storage systems. Thus, the inherent 4.1 C. (39.4xc2x0 F.) temperature limit is avoided by the present invention, through the use of a thermally stratified storage fluid with characteristics appropriate to lower temperature stratification.
Cooling can be accomplished in charging and discharging storage at temperatures low enough to produce combustion turbine inlet air as cold as, or even colder than, those available from ice thermal energy storage systems, thus maximizing combustion turbine power output enhancement. More importantly the present invention does so while preserving the relative simplicity of design and operation and the low unit capital cost associated with the conventional chilled water thermal energy storage, while avoiding the relative complexity, inefficiency, and higher unit capital cost of ice thermal energy storage systems.
Also, the lower supply temperature in storage results in a larger temperature differential within storage and, thus, in a smaller less expensive storage volume requirement. In fact, the storage volume will typically be significantly reduced from that required by either the conventional chilled water thermal energy storage approach or the weekly ice thermal storage approach.